There is a long felt need for an economically viable process to convert acetic acid to ethanol. Ethanol is an important commodity feedstock for a variety of industrial products and is also used as a fuel additive with gasoline. Ethanol can readily be dehydrated to ethylene, which can then be converted to a variety of products, both polymeric and small molecule-based. Ethanol is conventionally produced from feedstocks where price fluctuations are becoming more significant. That is, fluctuating natural gas and crude oil prices contribute to fluctuations in the cost of conventionally produced, petroleum, natural gas or corn or other agricultural product-sourced ethanol, making the need for alternative sources of ethanol all the greater when oil prices and/or agricultural product prices rise.
It has been reported that ethanol can be produced from the hydrogenation of acetic acid, but most of these processes feature several drawbacks for a commercial operation. For instance, U.S. Pat. No. 2,607,807 discloses that ethanol can be formed from acetic acid over a ruthenium catalyst at extremely high pressures of 700-950 bars in order to achieve yields of around 88%, whereas low yields of only about 40% are obtained at pressures of about 200 bar. Nevertheless, both of these conditions are unacceptable and uneconomical for a commercial operation.
More recently, it has been reported that ethanol can be produced from hydrogenating acetic acid using a cobalt catalyst again at superatmospheric pressures such as about 40 to 120 bar. See, for example, U.S. Pat. No. 4,517,391 to Shuster et al. However, the only example disclosed therein employs reaction pressure in the range of about 300 bar still making this process undesirable for a commercial operation. In addition, the process calls for a catalyst containing no less than 50 percent cobalt by weight plus one or more members selected from the group consisting of copper, manganese, molybdenum, chromium, and phosphoric acid, thus rendering the process economically non-viable. Although there is a disclosure of use of simple inert catalyst carriers to support the catalyst materials, there is no specific example of supported metal catalysts.
U.S. Pat. No. 5,149,680 to Kitson et al. describes a process for the catalytic hydrogenation of carboxylic acids and their anhydrides to alcohols and/or esters utilizing a platinum group metal alloy catalysts. The catalyst is comprised of an alloy of at least one noble metal of group VIII of the Periodic Table and at least one metal capable of alloying with the group VIII noble metal, admixed with a component comprising at least one of the metals rhenium, tungsten or molybdenum. Although it has been claimed therein that improved selectivity to alcohols are achieved over the prior art references it was still reported that 3 to 9 percent of alkanes, such as methane and ethane are formed as by-products during the hydrogenation of acetic acid to ethanol under their optimal catalyst conditions.
U.S. Pat. No. 4,777,303 to Kitson et al. describes a process for the productions of alcohols by the hydrogenation of carboxylic acids. The catalyst used in this case is a heterogeneous catalyst comprising a first component which is either molybdenum or tungsten and a second component which is a noble metal of Group VIII of the Periodic Table of the elements, optionally on a support, for example, a high surface area graphitized carbon. The selectivity to a combined mixture of alcohol and ester is reported to be only in the range of about 73 to 87 percent with low conversion of carboxylic acids at about 16 to 58 percent. In addition, no specific example of conversion of acetic acid to ethanol is provided.
U.S. Pat. No. 4,804,791 to Kitson et al. describes another process for the productions of alcohols by the hydrogenation of carboxylic acids. In this process, ethanol is produced from acetic acid or propanol is produced from propionic acid by contacting either acetic acid or propionic acid in the vapor phase with hydrogen at elevated temperature and a pressure in the range from 1 to 150 bar in the presence of a catalyst comprising as essential components (i) a noble metal of Group VIII of the Periodic Table of the elements, and (ii) rhenium, optionally on a support, for example a high surface area graphitized carbon. The conversion of acetic acid to ethanol ranged from 0.6% to 69% with selectivity to ethanol was in the range of about 6% to 97%.
From the foregoing it is apparent that existing processes do not have the requisite selectivity to ethanol or existing art employs catalysts, which are expensive and/or non-selective for the formation of ethanol and produces undesirable by-products.